The present invention relates to a surface-treated steel sheet excellent in corrosion resistance and formability applicable mainly for automobile body uses.
There is at present an increasing demand for improvement of both corrosion resistance and formability of steel sheets for automobile body uses. Particularly as to corrosion resistance, a problem is that pitting corrosion is produced in a joint portion between steel sheets known as a hem flange. Since painting, if any, does not cause the paint to adhere to the hem flange, a steel sheet is demanded to be corrosion-resistant for this portion in a non-painted state. For the purpose of improving corrosion resistance of steel sheet to satisfy this demand, a steel sheet manufactured by plating the steel sheet with a Znxe2x80x94Ni alloy of a thin coating weight of 20 to 30 g/m2, and further forming a chromate film and an organic film on the alloy film is now widely in use. While such a steel sheet has sufficient performance in corrosion resistance as well as in formability, the presence of an upper organic film acting as an insulating layer poses problems of easy occurrence of poor appearance upon ED-painting and difficulty to obtain a uniform appearance of painting. In addition, use of expensive nickel and containing detrimental hexavalent chromium are another problems. While a galvanized steel sheet having an increased coating weight or a Znxe2x80x94Fe alloy coated steel sheet is also used, an increase in coating weight of plating generally results in an improved corrosion resistance but in a poorer formability. It is therefore very difficult to satisfy requirements for both corrosion resistance and formability.
Japanese Examined Patent Publication No. 3-28509 discloses a highly corrosion-resistant plated steel sheet having a magnesium plating layer formed on a galvanizing layer, and Japanese Unexamined Patent Publication No. 2-254178 discloses a highly corrosion-resistant plated steel sheet having a composite film, comprising a metal magnesium and an oxide thereof, formed on a galvanizing layer. These steel sheets, having a high corrosion resistance, permit reduction of the coating weight, and an improvement to some extent is observed in formability, but has not as yet a performance sufficient to satisfy the general requirements.
(WO85/103089 and U.S. Pat. No. 4,722,753 describe corrosion-resistant coated metal objects and methods for producing the same by phosphate conversion coating, wherein said phosphate conversion coating is an improved zinc phosphate conversion coating method. The phosphating solution comprises first and second divalent cations, first metal cations selected from magnesium and transition metals having a hydroxide with lower solubility in alkaline solution than zinc hydroxide and zinc cations.)
The present invention has therefore an object to provide a coated steel sheet which solves the aforementioned drawbacks, satisfies requirements for both corrosion resistance and formability, and satisfies other basic properties required for a steel sheet mainly for automobile body uses, and a manufacturing method thereon.
In summary, the present invention provides:
(1) A surface-treated steel sheet comprising an amorphous inorganic film containing at least 5% magnesium and having a weight within a range of from 0.1 to 2.0 g/m2, formed on the surface of a zinc or zinc alloy plated steel sheet; wherein the inorganic film is soluble in an acidic solution and hardly soluble in a neutral or alkaline solution.
(2) A surface-treated steel sheet comprising a phosphate film formed on the surface of a zinc or zinc alloy plated steel sheet, and an amorphous inorganic film containing at least 5% magnesium and having a weight of at least 0.1 g/m2 formed on the phosphate film; wherein the inorganic film is soluble in an acidic solution and hardly soluble in a neutral or alkaline solution, and the inorganic film and the phosphate film have a total film weight of up to 2.0 g/m2.
(3) A surface-treated steel sheet according to item (2) above, wherein the phosphate film is a zinc phosphate film modified with one or more selected from the group consisting of nickel, magnesium, manganese, calcium, cobalt and copper.
(4) A surface-treated steel sheet according to item (3) above, wherein the amorphous inorganic film and the phosphate film have a total film weight within a range of from over 2.0 g/m2 to 3.0 g/m2.
(5) A surface-treated steel sheet according to any one of items (1) to (4) above, wherein the inorgarnic film comprises one or more selected from the group consisting of phosphoric acid, phosphates, biphosphates, condensed phosphoric acids, condensed phosphates, organic phosphoric acids, and organic phosphates.
(6) A surface-treated steel sheet according to any one of items (1) to (5) above, wherein a solution is coated onto the surface of the steel sheet having a clean surface; the steel sheet is a zinc or zinc alloy plated steel sheet or a zinc or zinc alloy plated steel sheet coated with a phosphate film; the aqueous solution contains magnesium dihydrogenphosphate as an essential component in a magnesium concentration in nonvolatile matters of at least 5%; and the steel sheet is baked at a temperature within a range of from 90 to 150xc2x0 C., and air-cooled.
The surface-treated steel sheet of the present invention comprises an amorphous inorganic film containing magnesium as an upper layer on a galvanized steel sheet, wherein this film is hardly soluble in a neutral or alkaline solution and soluble in an acidic solution.
Magnesium contained in the inorganic film has a function of stabilizing corrosion products of zinc, thereby inhibiting progress of rust, and is therefore primarily necessary for improving corrosion resistance.
The morphology of magnesium compound in the inorganic film also has an effect on corrosion resistance. Morphology of magnesium compound in a metallic form, while being favorable for corrosion resistance, poses a problem in formability as described later, and further, causes very difficult problems in manufacturing technology as well as in manufacturing cost. A film mainly comprising crystalline magnesium cannot give a sufficiently satisfactory corrosion resistance because of a high porosity. For these reasons, the most preferable morphology of magnesium is in an amorphous form which permits formation of a tight layer. Whether amorphous or not can be determined through observation of crystal by surface SEM and presence of diffraction patterns in an X-ray diffraction.
In order to improve formability, the inorganic film of the invention must be an amorphous film. A film comprising metallic magnesium, magnesium oxide or magnesium phosphate has not effect of improving formability. Particularly when the coating weight is increased, the resultant steel sheet cannot withstand high-speed pressing for automobile. The amorphous inorganic film covers the soft galvanizing layer to serve as a hard barrier film, thereby inhibiting flaking of the galvanizing layer. The film itself has an excellent lubricating effect. Further, even upon generation of heat from the steel sheet subjected to press forming, the film does not lose this excellent effect, thus giving a very good formability.
The amorphous inorganic film containing magnesium, serving as a barrier film against corrosive factors, is favorable for improving corrosion resistance. However, when the film acts as a barrier against reactions in the chemical conversion treatment (phosphate treatment) carried out in automotive coating, the chemical conversion film does not adhere, thus causing problems in coating appearance and paint adhesion. The inorganic film of the invention must necessarily be solved in a weak acidic solution environment of such a chemical conversion solution (usually having a pH within a range of from 2 to 3), and this is the very point of the invention. Being soluble in an acidic solution means that application of the aforementioned chemical conversion treatment does not cause an abnormality such as a phosphate coating defect. A part of magnesium dissolved in the chemical conversion solution is trapped in the resultant chemical conversion film, thus facilitating formation of a dense and corrosion-resistant magnesium-containing chemical conversion film. It is needless to mention that, even after the chemical conversion treatment, another part of magnesium remains insoluble and contributes to improvement of corrosion resistance.
On the other hand, the portion of an automobile body requiring the highest corrosion resistance is the joint portion of steel sheets known as a hem flange. The chemical conversion treatment solution cannot sufficiently penetrate into this portion. As a result, a high corrosion resistance cannot be ensured through the chemical conversion film alone. In contrast, the inorganic film of the invention remains substantially completely without being dissolved, and permits achievement of a high corrosion resistance.
The inorganic film of the invention must be soluble in an acidic solution, as described above. In order to achieve a high corrosion resistance at the hem flange, on the other hand, the inorganic film of the invention must be hardly soluble in a neutral or alkaline solution. The inorganic film, if soluble in a neutral or alkaline solution, would be poor in dew-point corrosion resistance during storage, and easily dissolved in an alkaline degreasing solution on an automobile coating line, thus failing to have a corrosion resistance improving effect. A low solubility in a neutral or alkaline solution means that the film remains even through an alkaline degreasing process as described above.
It is more preferable to apply a zinc phosphate chemical conversion treatment with zinc phosphate or modified zinc phosphate to the galvanizing layer to form thereon an amorphous inorganic film of the invention. The amorphous inorganic film is held in zinc phosphate intercrystalline gaps, thus further improving resistance to an neutral or alkaline solution while maintaining phosphatability on the automobile coating line.
The term xe2x80x9cbeing amorphousxe2x80x9d as used in a case where a zinc phosphate chemical conversion treatment is applied onto a galvanizing layer to form thereon an amorphous inorganic film shall mean that there is observed no crystals caused by the inorganic film (for example, a magnesium biphosphate film) via a surface SEM observation and diffraction pattern observation in an X-ray diffraction, and only crystals of the steel sheet substrate, and/or crystals of the galvanizing layer, and/or crystals resulting from the zinc phosphate chemical conversion treatment are observed. The amorphous state can be determined via such means.
It is not desirable that the amorphous inorganic film of the invention contains compounds which may impair phosphatability such as chromium compounds or aluminum compounds. The amorphous inorganic film should preferably comprise phosphoric acid, a phosphate, a biphosphate, a condensed phosphoric acid, a condensed phosphate, organic phosphoric acid or an organic phosphate, containing magnesium, but the components are not limited to those enumerated above. A film comprising silica sol or a silicate is not desirable because it is poor in solubillty in a weak acidic solution and impairs paintability.
The magnesium content in the amorphous inorganic film of the invention must be at least 5%. A magnesium content of under 5% is not desirable in terms of corrosion resistance. A phosphoric acid amorphous inorganic film has usually a magnesium content of about 10%, but this is not limitative. A magnesium content of 100% corresponds to metallic magnesium, and is not of course desirable as described above.
The coating weight of the amorphous inorganic film of the invention must be within a range of from 0.1 to 2.0 g/m2. A coating weight of under 0.1 g/m2 gives no improving effect of corrosion resistance and formability. A coating weight of over 2.0 g/m2 results in poorer formability and weldability. In a more preferred embodiment of the present invention, in which the amorphous inorganic film is formed, via a phosphate film, on the galvanizing layer, the upper limit of the film weight must be up to 2.0 g/m2 in total of the phosphate film and the amorphous inorganic film. A film weight of over this level leads o poorer formability and weldability.
In a further more preferred embodiment of the invention, an amorphous inorganic film which is soluble in an acidic solution, hardly soluble in a neutral or alkaline solution and contains at least 5% magnesium is formed via a phosphate film modified with one or more selected from the group consisting of nickel, magnesium, manganese, calcium, cobalt and copper. This further improves corrosion resistance, and even an increased coating weight leads to a smaller extent of deterioration of formability and weldabillity. That is, the film weight in this case is limited to an upper limit of a total of 3.0 g/m2 of the undercoat modified zinc phosphate film and the amorphous inorganic film. Sufficient weldability and formability can be ensured so far as this upper limit is not exceeded. The term the zinc phosphate film modified with nickel, magnesium, manganese, calcium, cobalt and/or copper as used herein shall mean a chemical conversion film formed with a zinc phosphate treatment solution in which ions of nickel, magnesium, manganese, calcium, cobalt and/or copper are co-existent. Only a very slight part of zinc in the zinc phosphate crystals (hopeite: Zn3(PO4)24H2O) is considered to be replaced by other metals, whereas diffraction patterns available from X-ray diffraction thereof cannot be discriminated from those of hopeite. Nickel, magnesium, manganese, calcium, cobalt and/or copper accounts for several % in total weight in the zinc phosphate film.
The aforementioned amorphous inorganic film which is hardly soluble in a neutral or alkaline solution, soluble in an acidic solution and contains magnesium may be prepared by a simple method at a low cost. There is available, for example, a method of coating an acidic solution containing magnesium biphosphate (magnesium dihydrogenphosphate, also known as primary magnesium phosphate) and baking the same. Coating may be carried out by any of the means commonly used such as spraying, dipping and use of a roll coater, and the coating method is not limited to a particular one.
There is no particular limitation imposed on the concentration of magnesium dihydrogenphosphate in the solution to be coated. Magnesium biphosphate (magnesium dihydrogenphosphate) solution commercially available at present has a concentration of 50% a method of using such a solution by appropriately diluting so as to achieve a prescribed coating weight is preferable. Magnesium should have a concentration of at least 5% in nonvolatile matters in the solution. With a lower magnesium concentration, it is impossible to obtain a magnesium concentration in the formed film of at least a prescribed value, leading to an insufficient corrosion resistance.
The solution contains magnesium biphosphate (magnesium dihydrogenphosphate) as an essential component, and phosphoric acid, condensed phosphoric acid, organic phosphoric acid or any of various phosphates should preferably be added. This addition makes it possible to control physical properties such as viscosity of the solution to values suitable for coating conditions. Even when adding these additives, it is necessary to adjust the magnesium content in nonvolatile matters in the solution to a value of at least 5%.
The other phosphates containing magnesium (for example, MgHPO4 or Mg3(PO4)2) are very hardly soluble in water, it is difficult to coat a solution of these salts. It is however possible to dissolved the same in a slight amount by adding an acid such as phosphoric acid in excess. In this case, however, the magnesium concentration in the resultant film is far lower than 5%, and an improving effect of corrosion resistance is unavailable. When coating an aqueous suspension prepared by dispersion-adjusting these low-solubility salts by the use of a dispersant such as starch or dextrin, the film is in crystalline state with a poor adhesion to the substrate.
Conditions for baking the steel sheet after coating the acidic solution containing magnesium biphosphate (magnesium dihydrogenphosphate) onto the steel sheet are also very important. It is essential to bake the steel sheet so as to achieve a temperature within a range of from 90 to 150xc2x0 C. immediately after coating with the solution. At a temperature of under 90xc2x0 C., the resultant film would have a poorer water-proof property. A temperature of over 150xc2x0 C. impairs, on the other hand, solubility in a weak acidic solution. Baking should be carried out immediately after coating. If not, there occur reactions between acidic components in the solution and zinc and the like on the galvanizing surface, and this causes growth of a brittle crystalline film.
After baking, the baked steel sheet must be air-cooled (including spontaneous cooling by holding). For example, water spraying causes partial dissolution of the film, tending to result in a poor appearance. The surface before treatment should be clean. Coating on a surface containing stain makes it impossible to obtain a normal film.
When forming a phosphate film on the surface of the galvanized steel sheet, and further forming thereon an inorganic film of the invention, it suffices first to apply a zinc phosphate chemical conversion treatment to the galvanized steel sheet by a known method and coat the inorganic film by the method as described above. Prior to the zinc phosphate chemical conversion treatment, there may be carried out a surface adjustment (treatment with titanium colloid, and/or a treatment with an acid solution, and/or surface activation through brush polishing) by any of known methods.